Communication towers include signal processing equipment that is particularly susceptible to noise generated from within the power system and grounding system for the tower. Improper or poor electrical grounding can create personnel safety problems as well as interference with signal processing equipment. Communication towers are generally equipped with power supplies for signal communications, controls, lighting, air conditioning and auxiliary equipment that may introduce transient voltages into the communications signals, particularly if not properly grounded. Furthermore, tall towers attract lightning. The high current of a lightning strike can cause extreme voltage changes if the grounding system does not have a low resistance to earth ground.
The National Electrical Code® 2005 Edition (NFPA 70) (Code) defines the terms “grounding electrode”, “grounding electrode conductor” and “grounding conductor” as parts of the tightly connected system referred to here simply as the “local ground”. The Code also defines “grounded conductor” as the line referred to here as the “utility neutral”. The local ground and utility neutral should be electrically connected at a single point, defined by the Code as the “main bonding jumper”. This jumper ideally is a single screw that can be removed to isolate the utility neutral from the local ground.)
Generally, as a safety measure, or in order to eliminate sources of interference, it is necessary to test the grounding system of a communication tower periodically to ensure that there is still a low resistance between the communication tower and earth ground. In order to ensure the accuracy of the ground resistance measurements, it is necessary to eliminate possible alternate ground paths or circuits that may falsely indicate good grounding system measurements. For example, the utility AC power system neutral is normally connected to earth ground at multiple sites across its distribution network. If the ground system for the utility distribution system provides a resistance comparable to or lower than that of the local ground, a ground current measuring device will measure the lower resistance of the two systems in parallel, rather than the actual resistance of the local ground. It is therefore necessary to isolate the local grounding system from the utility neutral grounding system during testing.
The resistance of local grounding systems is commonly tested using a ground resistance meter and the “fall of potential” method. Two auxiliary electrodes are driven into the soil at significant distances along a straight line away from the ground being tested. During a normal test of a ground rod (the simplest type of local ground) the meter supplies a specific pulsating current between the ground rod under test and the most remote auxiliary electrode. A series of measurements of the voltage drops between the ground rod under test and the intermediate electrode are made by moving the intermediate electrode in steps away from the ground rod under test. The most common equipment used in the fall of potential method generates a 128 Hz pulsating current between the local ground rod and the distant auxiliary electrode. Attempts to use bandpass filters to permit 60 Hz current to flow through the grounding circuit while blocking 128 Hz current feedback through the electrical utility have been unable to filter out all of the test frequency signal. During the fall of potential test the utility neutral must disconnected from the ground rod, as the current flow on the neutral will cause an inaccurate reading.
Currently, the methods employed to measure the resistance of local grounding systems at communication towers require that the utility neutral be completely disconnected from the local ground for the communication tower and associated equipment. This eliminates the redundancy of independent grounding systems, and creates an unacceptable risk for communications companies. This lack of redundancy creates a potential danger to personnel and equipment due to the possibility of potential differences between the two grounding systems. Possible high voltage transients present a danger of electrical shock to personnel, who may inadvertently come into contact between the neutral bus and ground during a grounding system test, forming a replacement connection for the open ground to neutral connection. Similarly, sensitive communications equipment power supplies are at risk of exposure to voltage and current in excess of their rated voltage and current due to a floating neutral voltage.
Therefore there is a need for a ground testing method and device that provides effective electrical ground isolation from the utility neutral for the ground resistance meter's signals, and simultaneously provides a grounding path capable of carrying enough current to trip circuit breakers in the event of a “hot” wire shorting to a floating neutral.